docs: def filter_select_columns_intensity(df, prefix, columns):
      """
      Filter dataframe to include specified columns, retaining any Intensity columns.
      """
      # Note: I use %s.+ (not %s.*) so it forces a match with the prefix string, ONLY if it is followed by something.
      return df.filter(regex='^(%s.+|%s)$' % (prefix, '|'.join(columns)) )